Describe the mechanism of action.
Nonspecific PDE inhibition → ↓ hydrolysis of cAMP → ↑ cAMP levels
Adenosine receptor blockade
Describe the desired effect.
Inhibition of proinflammatory mediators
Deceleration of fibrotic changes in the lung
Relaxation of the bronchial musculature → bronchodilation
List adverse effects of phosphodiesterase type 5 inhibitors.
Headaches, cutaneous flushing
Sildenafil: visual deficits (blue-tinted vision, or cyanopia) due to PDE-6 inhibition in the retina 
Runny nose, nasal congestion
Rarely: myocardial infarction, stroke, hearing loss, optic neuropathy
List adverse effects of phosphodiesterase type 4 inhibitors.
GI upset (nausea, abdominal pain)
Mental disorders: sleep disturbances, anxiety, depression
List adverse effects of phosphodiesterase type 3 inhibitors.
Tachycardia, ventricular arrhythmias (most common and severe side effect, so not recommended for chronic use)